Pull request: 3443 dhcp broadcast vol.2

Merge in DNS/adguard-home from 3443-dhcp-broadcast-vol.2 to master Closes #3443. Squashed commit of the following: commit a85af89cb43f2489126fe3c12366fc034e89f59d Merge: 72eb3a88 a4e07827 Author: Eugene Burkov <E.Burkov@AdGuard.COM> Date: Thu Sep 30 18:08:19 2021 +0300 Merge branch 'master' into 3443-dhcp-broadcast-vol.2 commit 72eb3a8853540b06ee1096decf50e836b539fe45 Author: Eugene Burkov <E.Burkov@AdGuard.COM> Date: Thu Sep 30 18:03:19 2021 +0300 dhcpd: imp code readability commit 2d1fbc40d04a4125855d6be9f02e09d15430150d Author: Eugene Burkov <E.Burkov@AdGuard.COM> Date: Thu Sep 30 14:16:59 2021 +0300 dhcpd: imp tests commit 889fad3084ad2b81edfc12100e2ce29d323227ba Author: Eugene Burkov <E.Burkov@AdGuard.COM> Date: Wed Sep 29 20:09:25 2021 +0300 dhcpd: imp code, docs commit 1fd6b2346ff66e033bceaa169aed751be5822ca8 Author: Eugene Burkov <E.Burkov@AdGuard.COM> Date: Thu Sep 23 16:08:18 2021 +0300 dhcpd: unicast to mac address
2021-09-30 18:28:19 +03:00 · 2021-09-30 18:28:19 +03:00 · da86620288
parent a4e078271c
commit da86620288
12 changed files with 531 additions and 231 deletions
--- a/go.mod
+++ b/go.mod
@ -12,12 +12,14 @@ require (
 	github.com/fsnotify/fsnotify v1.4.9
 	github.com/go-ping/ping v0.0.0-20210506233800-ff8be3320020
 	github.com/google/go-cmp v0.5.5
+	github.com/google/gopacket v1.1.19
 	github.com/google/renameio v1.0.1
 	github.com/insomniacslk/dhcp v0.0.0-20210310193751-cfd4d47082c2
 	github.com/kardianos/service v1.2.0
 	github.com/lucas-clemente/quic-go v0.21.1
+	github.com/mdlayher/ethernet v0.0.0-20190606142754-0394541c37b7
 	github.com/mdlayher/netlink v1.4.0
-	github.com/mdlayher/raw v0.0.0-20210412142147-51b895745faf // indirect
+	github.com/mdlayher/raw v0.0.0-20210412142147-51b895745faf
 	github.com/miekg/dns v1.1.43
 	github.com/satori/go.uuid v1.2.0
 	github.com/stretchr/objx v0.1.1 // indirect
--- a/go.sum
+++ b/go.sum
@ -93,6 +93,8 @@ github.com/google/go-cmp v0.5.5 h1:Khx7svrCpmxxtHBq5j2mp/xVjsi8hQMfNLvJFAlrGgU=
 github.com/google/go-cmp v0.5.5/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
 github.com/google/go-github v17.0.0+incompatible/go.mod h1:zLgOLi98H3fifZn+44m+umXrS52loVEgC2AApnigrVQ=
 github.com/google/go-querystring v1.0.0/go.mod h1:odCYkC5MyYFN7vkCjXpyrEuKhc/BUO6wN/zVPAxq5ck=
+github.com/google/gopacket v1.1.19 h1:ves8RnFZPGiFnTS0uPQStjwru6uO6h+nlr9j6fL7kF8=
+github.com/google/gopacket v1.1.19/go.mod h1:iJ8V8n6KS+z2U1A8pUwu8bW5SyEMkXJB8Yo/Vo+TKTo=
 github.com/google/martian v2.1.0+incompatible/go.mod h1:9I4somxYTbIHy5NJKHRl3wXiIaQGbYVAs8BPL6v8lEs=
 github.com/google/pprof v0.0.0-20181206194817-3ea8567a2e57/go.mod h1:zfwlbNMJ+OItoe0UupaVj+oy1omPYYDuagoSzA8v9mc=
 github.com/google/renameio v1.0.1 h1:Lh/jXZmvZxb0BBeSY5VKEfidcbcbenKjZFzM/q0fSeU=
@ -268,6 +270,7 @@ golang.org/x/exp v0.0.0-20190121172915-509febef88a4/go.mod h1:CJ0aWSM057203Lf6IL
 golang.org/x/lint v0.0.0-20180702182130-06c8688daad7/go.mod h1:UVdnD1Gm6xHRNCYTkRU2/jEulfH38KcIWyp/GAMgvoE=
 golang.org/x/lint v0.0.0-20181026193005-c67002cb31c3/go.mod h1:UVdnD1Gm6xHRNCYTkRU2/jEulfH38KcIWyp/GAMgvoE=
 golang.org/x/lint v0.0.0-20190227174305-5b3e6a55c961/go.mod h1:wehouNa3lNwaWXcvxsM5YxQ5yQlVC4a0KAMCusXpPoU=
+golang.org/x/lint v0.0.0-20200302205851-738671d3881b/go.mod h1:3xt1FjdF8hUf6vQPIChWIBhFzV8gjjsPE/fR3IyQdNY=
 golang.org/x/mod v0.1.1-0.20191105210325-c90efee705ee/go.mod h1:QqPTAvyqsEbceGzBzNggFXnrqF1CaUcvgkdR5Ot7KZg=
 golang.org/x/mod v0.4.2/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/net v0.0.0-20180724234803-3673e40ba225/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
@ -377,6 +380,7 @@ golang.org/x/tools v0.0.0-20190328211700-ab21143f2384/go.mod h1:LCzVGOaR6xXOjkQ3
 golang.org/x/tools v0.0.0-20190425150028-36563e24a262/go.mod h1:RgjU9mgBXZiqYHBnxXauZ1Gv1EHHAz9KjViQ78xBX0Q=
 golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/tools v0.0.0-20191216052735-49a3e744a425/go.mod h1:TB2adYChydJhpapKDTa4BR/hXlZSLoq2Wpct/0txZ28=
+golang.org/x/tools v0.0.0-20200130002326-2f3ba24bd6e7/go.mod h1:TB2adYChydJhpapKDTa4BR/hXlZSLoq2Wpct/0txZ28=
 golang.org/x/tools v0.1.1/go.mod h1:o0xws9oXOQQZyjljx8fwUC0k7L1pTE6eaCbjGeHmOkk=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20191011141410-1b5146add898/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
--- a/internal/dhcpd/broadcast_bsd.go
+++ b/internal/dhcpd/broadcast_bsd.go
@ -5,33 +5,17 @@ package dhcpd

 import (
 	"net"
-
-	"github.com/AdguardTeam/golibs/log"
-	"github.com/insomniacslk/dhcp/dhcpv4"
 )

 // broadcast sends resp to the broadcast address specific for network interface.
-func (s *v4Server) broadcast(peer net.Addr, conn net.PacketConn, resp *dhcpv4.DHCPv4) {
-	// peer is expected to be of type *net.UDPConn as the server4.NewServer
-	// initializes it.
-	udpPeer, ok := peer.(*net.UDPAddr)
-	if !ok {
-		log.Error("dhcpv4: peer is of unexpected type %T", peer)
-
-		return
-	}
-
+func (c *dhcpConn) broadcast(respData []byte, peer *net.UDPAddr) (n int, err error) {
 	// Despite the fact that server4.NewIPv4UDPConn explicitly sets socket
 	// options to allow broadcasting, it also binds the connection to a
-	// specific interface.  On FreeBSD and OpenBSD conn.WriteTo causes
-	// errors while writing to the addresses that belong to another
-	// interface.  So, use the broadcast address specific for the binded
-	// interface.
-	udpPeer.IP = s.conf.broadcastIP
+	// specific interface.  On FreeBSD and OpenBSD net.UDPConn.WriteTo
+	// causes errors while writing to the addresses that belong to another
+	// interface.  So, use the broadcast address specific for the interface
+	// bound.
+	peer.IP = c.bcastIP

-	log.Debug("dhcpv4: sending to %s: %s", peer, resp.Summary())
-
-	if _, err := conn.WriteTo(resp.ToBytes(), peer); err != nil {
-		log.Error("dhcpv4: conn.Write to %s failed: %s", peer, err)
-	}
+	return c.udpConn.WriteTo(respData, peer)
 }
--- a/internal/dhcpd/broadcast_bsd_test.go
+++ b/internal/dhcpd/broadcast_bsd_test.go
@ -8,17 +8,16 @@ import (
 	"net"
 	"testing"

-	"github.com/AdguardTeam/golibs/netutil"
 	"github.com/insomniacslk/dhcp/dhcpv4"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )

-func TestV4Server_Send_broadcast(t *testing.T) {
+func TestDHCPConn_Broadcast(t *testing.T) {
 	b := &bytes.Buffer{}
 	var peer *net.UDPAddr

-	conn := &fakePacketConn{
+	udpConn := &fakePacketConn{
 		writeTo: func(p []byte, addr net.Addr) (n int, err error) {
 			udpPeer, ok := addr.(*net.UDPAddr)
 			require.True(t, ok)
@ -31,57 +30,22 @@ func TestV4Server_Send_broadcast(t *testing.T) {
 			return n, nil
 		},
 	}
-
+	conn := &dhcpConn{
+		udpConn: udpConn,
+		bcastIP: net.IP{1, 2, 3, 255},
+	}
 	defaultPeer := &net.UDPAddr{
 		IP: net.IP{1, 2, 3, 4},
 		// Use neither client nor server port.
 		Port: 1234,
 	}
-	s := &v4Server{
-		conf: V4ServerConf{
-			broadcastIP: net.IP{1, 2, 3, 255},
-		},
-	}
+	respData := (&dhcpv4.DHCPv4{}).ToBytes()

-	testCases := []struct {
-		name string
-		req  *dhcpv4.DHCPv4
-		resp *dhcpv4.DHCPv4
-	}{{
-		name: "nak",
-		req: &dhcpv4.DHCPv4{
-			GatewayIPAddr: netutil.IPv4Zero(),
-		},
-		resp: &dhcpv4.DHCPv4{
-			Options: dhcpv4.OptionsFromList(
-				dhcpv4.OptMessageType(dhcpv4.MessageTypeNak),
-			),
-		},
-	}, {
-		name: "fully_unspecified",
-		req: &dhcpv4.DHCPv4{
-			GatewayIPAddr: netutil.IPv4Zero(),
-			ClientIPAddr:  netutil.IPv4Zero(),
-		},
-		resp: &dhcpv4.DHCPv4{
-			Options: dhcpv4.OptionsFromList(
-				dhcpv4.OptMessageType(dhcpv4.MessageTypeOffer),
-			),
-		},
-	}}
+	_, _ = conn.broadcast(respData, cloneUDPAddr(defaultPeer))

-	for _, tc := range testCases {
-		t.Run(tc.name, func(t *testing.T) {
-			s.send(cloneUDPAddr(defaultPeer), conn, tc.req, tc.resp)
-			assert.EqualValues(t, tc.resp.ToBytes(), b.Bytes())
+	assert.EqualValues(t, respData, b.Bytes())
 	assert.Equal(t, &net.UDPAddr{
-				IP:   s.conf.broadcastIP,
+		IP:   conn.bcastIP,
 		Port: defaultPeer.Port,
-				Zone: defaultPeer.Zone,
 	}, peer)
-		})
-
-		b.Reset()
-		peer = nil
-	}
 }
--- a/internal/dhcpd/broadcast_others.go
+++ b/internal/dhcpd/broadcast_others.go
@ -5,17 +5,10 @@ package dhcpd

 import (
 	"net"
-
-	"github.com/AdguardTeam/golibs/log"
-	"github.com/insomniacslk/dhcp/dhcpv4"
 )

 // broadcast sends resp to the broadcast address specific for network interface.
-func (s *v4Server) broadcast(peer net.Addr, conn net.PacketConn, resp *dhcpv4.DHCPv4) {
-	respData := resp.ToBytes()
-
-	log.Debug("dhcpv4: sending to %s: %s", peer, resp.Summary())
-
+func (c *dhcpConn) broadcast(respData []byte, peer *net.UDPAddr) (n int, err error) {
 	// This write to 0xffffffff reverts some behavior changes made in
 	// https://github.com/AdguardTeam/AdGuardHome/issues/3289.  The DHCP
 	// server should broadcast the message to 0xffffffff but it's
@ -26,26 +19,13 @@ func (s *v4Server) broadcast(peer net.Addr, conn net.PacketConn, resp *dhcpv4.DH
 	// https://github.com/AdguardTeam/AdGuardHome/issues/3366.
 	//
 	// See also https://github.com/AdguardTeam/AdGuardHome/issues/3539.
-	if _, err := conn.WriteTo(respData, peer); err != nil {
-		log.Error("dhcpv4: conn.Write to %s failed: %s", peer, err)
-	}
-
-	// peer is expected to be of type *net.UDPConn as the server4.NewServer
-	// initializes it.
-	udpPeer, ok := peer.(*net.UDPAddr)
-	if !ok {
-		log.Error("dhcpv4: peer is of unexpected type %T", peer)
-
-		return
+	if n, err = c.udpConn.WriteTo(respData, peer); err != nil {
+		return n, err
 	}

 	// Broadcast the message one more time using the interface-specific
 	// broadcast address.
-	udpPeer.IP = s.conf.broadcastIP
+	peer.IP = c.bcastIP

-	log.Debug("dhcpv4: sending to %s: %s", peer, resp.Summary())
-
-	if _, err := conn.WriteTo(respData, peer); err != nil {
-		log.Error("dhcpv4: conn.Write to %s failed: %s", peer, err)
-	}
+	return c.udpConn.WriteTo(respData, peer)
 }
--- a/internal/dhcpd/broadcast_others_test.go
+++ b/internal/dhcpd/broadcast_others_test.go
@ -8,17 +8,16 @@ import (
 	"net"
 	"testing"

-	"github.com/AdguardTeam/golibs/netutil"
 	"github.com/insomniacslk/dhcp/dhcpv4"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )

-func TestV4Server_Send_broadcast(t *testing.T) {
+func TestDHCPConn_Broadcast(t *testing.T) {
 	b := &bytes.Buffer{}
 	var peers []*net.UDPAddr

-	conn := &fakePacketConn{
+	udpConn := &fakePacketConn{
 		writeTo: func(p []byte, addr net.Addr) (n int, err error) {
 			udpPeer, ok := addr.(*net.UDPAddr)
 			require.True(t, ok)
@ -31,66 +30,27 @@ func TestV4Server_Send_broadcast(t *testing.T) {
 			return n, nil
 		},
 	}
-
+	conn := &dhcpConn{
+		udpConn: udpConn,
+		bcastIP: net.IP{1, 2, 3, 255},
+	}
 	defaultPeer := &net.UDPAddr{
 		IP: net.IP{1, 2, 3, 4},
 		// Use neither client nor server port.
 		Port: 1234,
 	}
-	s := &v4Server{
-		conf: V4ServerConf{
-			broadcastIP: net.IP{1, 2, 3, 255},
-		},
-	}
+	respData := (&dhcpv4.DHCPv4{}).ToBytes()

-	testCases := []struct {
-		name string
-		req  *dhcpv4.DHCPv4
-		resp *dhcpv4.DHCPv4
-	}{{
-		name: "nak",
-		req: &dhcpv4.DHCPv4{
-			GatewayIPAddr: netutil.IPv4Zero(),
-		},
-		resp: &dhcpv4.DHCPv4{
-			Options: dhcpv4.OptionsFromList(
-				dhcpv4.OptMessageType(dhcpv4.MessageTypeNak),
-			),
-		},
-	}, {
-		name: "fully_unspecified",
-		req: &dhcpv4.DHCPv4{
-			GatewayIPAddr: netutil.IPv4Zero(),
-			ClientIPAddr:  netutil.IPv4Zero(),
-		},
-		resp: &dhcpv4.DHCPv4{
-			Options: dhcpv4.OptionsFromList(
-				dhcpv4.OptMessageType(dhcpv4.MessageTypeOffer),
-			),
-		},
-	}}
+	_, _ = conn.broadcast(respData, cloneUDPAddr(defaultPeer))

-	for _, tc := range testCases {
-		t.Run(tc.name, func(t *testing.T) {
-			s.send(cloneUDPAddr(defaultPeer), conn, tc.req, tc.resp)
-
-			// The same response is written twice.
-			respData := tc.resp.ToBytes()
+	// The same response is written twice but for different peers.
 	assert.EqualValues(t, append(respData, respData...), b.Bytes())

 	require.Len(t, peers, 2)

+	assert.Equal(t, cloneUDPAddr(defaultPeer), peers[0])
 	assert.Equal(t, &net.UDPAddr{
-				IP:   defaultPeer.IP,
-				Port: defaultPeer.Port,
-			}, peers[0])
-			assert.Equal(t, &net.UDPAddr{
-				IP:   s.conf.broadcastIP,
+		IP:   conn.bcastIP,
 		Port: defaultPeer.Port,
 	}, peers[1])
-		})
-
-		b.Reset()
-		peers = nil
-	}
 }
--- a/internal/dhcpd/conn_unix.go
+++ b/internal/dhcpd/conn_unix.go
@ -0,0 +1,244 @@
+//go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package dhcpd
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"time"
+
+	"github.com/AdguardTeam/golibs/errors"
+	"github.com/AdguardTeam/golibs/netutil"
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	"github.com/insomniacslk/dhcp/dhcpv4"
+	"github.com/insomniacslk/dhcp/dhcpv4/server4"
+	"github.com/mdlayher/ethernet"
+	"github.com/mdlayher/raw"
+)
+
+// dhcpUnicastAddr is the combination of MAC and IP addresses for responding to
+// the unconfigured host.
+type dhcpUnicastAddr struct {
+	// raw.Addr is embedded here to make *dhcpUcastAddr a net.Addr without
+	// actually implementing all methods.  It also contains the client's
+	// hardware address.
+	raw.Addr
+
+	// yiaddr is an IP address just allocated by server for the host.
+	yiaddr net.IP
+}
+
+// dhcpConn is the net.PacketConn capable of handling both net.UDPAddr and
+// net.HardwareAddr.
+type dhcpConn struct {
+	// udpConn is the connection for UDP addresses.
+	udpConn net.PacketConn
+	// bcastIP is the broadcast address specific for the configured
+	// interface's subnet.
+	bcastIP net.IP
+
+	// rawConn is the connection for MAC addresses.
+	rawConn net.PacketConn
+	// srcMAC is the hardware address of the configured network interface.
+	srcMAC net.HardwareAddr
+	// srcIP is the IP address  of the configured network interface.
+	srcIP net.IP
+}
+
+// newDHCPConn creates the special connection for DHCP server.
+func (s *v4Server) newDHCPConn(ifi *net.Interface) (c net.PacketConn, err error) {
+	// Create the raw connection.
+	var ucast net.PacketConn
+	if ucast, err = raw.ListenPacket(ifi, uint16(ethernet.EtherTypeIPv4), nil); err != nil {
+		return nil, fmt.Errorf("creating raw udp connection: %w", err)
+	}
+
+	// Create the UDP connection.
+	var bcast net.PacketConn
+	bcast, err = server4.NewIPv4UDPConn(ifi.Name, &net.UDPAddr{
+		// TODO(e.burkov):  Listening on zeroes makes the server handle
+		// requests from all the interfaces.  Inspect the ways to
+		// specify the interface-specific listening addresses.
+		//
+		// See https://github.com/AdguardTeam/AdGuardHome/issues/3539.
+		IP:   net.IP{0, 0, 0, 0},
+		Port: dhcpv4.ServerPort,
+	})
+	if err != nil {
+		return nil, fmt.Errorf("creating ipv4 udp connection: %w", err)
+	}
+
+	return &dhcpConn{
+		udpConn: bcast,
+		bcastIP: s.conf.broadcastIP,
+		rawConn: ucast,
+		srcMAC:  ifi.HardwareAddr,
+		srcIP:   s.conf.dnsIPAddrs[0],
+	}, nil
+}
+
+// wrapErrs is a helper to wrap the errors from two independent underlying
+// connections.
+func (c *dhcpConn) wrapErrs(action string, udpConnErr, rawConnErr error) (err error) {
+	switch {
+	case udpConnErr != nil && rawConnErr != nil:
+		return errors.List(fmt.Sprintf("%s both connections", action), udpConnErr, rawConnErr)
+	case udpConnErr != nil:
+		return fmt.Errorf("%s udp connection: %w", action, udpConnErr)
+	case rawConnErr != nil:
+		return fmt.Errorf("%s raw connection: %w", action, rawConnErr)
+	default:
+		return nil
+	}
+}
+
+// WriteTo implements net.PacketConn for *dhcpConn.  It selects the underlying
+// connection to write to based on the type of addr.
+func (c *dhcpConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
+	switch addr := addr.(type) {
+	case *dhcpUnicastAddr:
+		// Unicast the message to the client's MAC address.  Use the raw
+		// connection.
+		//
+		// Note: unicasting is performed on the only network interface
+		// that is configured.  For now it may be not what users expect
+		// so additionally broadcast the message via UDP connection.
+		//
+		// See https://github.com/AdguardTeam/AdGuardHome/issues/3539.
+		var rerr error
+		n, rerr = c.unicast(p, addr)
+
+		_, uerr := c.broadcast(p, &net.UDPAddr{
+			IP:   netutil.IPv4bcast(),
+			Port: dhcpv4.ClientPort,
+		})
+
+		return n, c.wrapErrs("writing to", uerr, rerr)
+	case *net.UDPAddr:
+		if addr.IP.Equal(net.IPv4bcast) {
+			// Broadcast the message for the client which supports
+			// it.  Use the UDP connection.
+			return c.broadcast(p, addr)
+		}
+
+		// Unicast the message to the client's IP address.  Use the UDP
+		// connection.
+		return c.udpConn.WriteTo(p, addr)
+	default:
+		return 0, fmt.Errorf("peer is of unexpected type %T", addr)
+	}
+}
+
+// ReadFrom implements net.PacketConn for *dhcpConn.
+func (c *dhcpConn) ReadFrom(p []byte) (n int, addr net.Addr, err error) {
+	return c.udpConn.ReadFrom(p)
+}
+
+// unicast wraps respData with required frames and writes it to the peer.
+func (c *dhcpConn) unicast(respData []byte, peer *dhcpUnicastAddr) (n int, err error) {
+	var data []byte
+	data, err = c.buildEtherPkt(respData, peer)
+	if err != nil {
+		return 0, err
+	}
+
+	return c.rawConn.WriteTo(data, &peer.Addr)
+}
+
+// Close implements net.PacketConn for *dhcpConn.
+func (c *dhcpConn) Close() (err error) {
+	rerr := c.rawConn.Close()
+	if errors.Is(rerr, os.ErrClosed) {
+		// Ignore the error since the actual file is closed already.
+		rerr = nil
+	}
+
+	return c.wrapErrs("closing", c.udpConn.Close(), rerr)
+}
+
+// LocalAddr implements net.PacketConn for *dhcpConn.
+func (c *dhcpConn) LocalAddr() (a net.Addr) {
+	return c.udpConn.LocalAddr()
+}
+
+// SetDeadline implements net.PacketConn for *dhcpConn.
+func (c *dhcpConn) SetDeadline(t time.Time) (err error) {
+	return c.wrapErrs("setting deadline on", c.udpConn.SetDeadline(t), c.rawConn.SetDeadline(t))
+}
+
+// SetReadDeadline implements net.PacketConn for *dhcpConn.
+func (c *dhcpConn) SetReadDeadline(t time.Time) error {
+	return c.wrapErrs(
+		"setting reading deadline on",
+		c.udpConn.SetReadDeadline(t),
+		c.rawConn.SetReadDeadline(t),
+	)
+}
+
+// SetWriteDeadline implements net.PacketConn for *dhcpConn.
+func (c *dhcpConn) SetWriteDeadline(t time.Time) error {
+	return c.wrapErrs(
+		"setting writing deadline on",
+		c.udpConn.SetWriteDeadline(t),
+		c.rawConn.SetWriteDeadline(t),
+	)
+}
+
+// ipv4DefaultTTL is the default Time to Live value as recommended by
+// RFC-1700 (https://datatracker.ietf.org/doc/html/rfc1700) in seconds.
+const ipv4DefaultTTL = 64
+
+// errInvalidPktDHCP is returned when the provided payload is not a valid DHCP
+// packet.
+const errInvalidPktDHCP errors.Error = "packet is not a valid dhcp packet"
+
+// buildEtherPkt wraps the payload with IPv4, UDP and Ethernet frames.  The
+// payload is expected to be an encoded DHCP packet.
+func (c *dhcpConn) buildEtherPkt(payload []byte, peer *dhcpUnicastAddr) (pkt []byte, err error) {
+	dhcpLayer := gopacket.NewPacket(payload, layers.LayerTypeDHCPv4, gopacket.DecodeOptions{
+		NoCopy: true,
+	}).Layer(layers.LayerTypeDHCPv4)
+
+	// Check if the decoding succeeded and the resulting layer doesn't
+	// contain any errors.  It should guarantee panic-safe converting of the
+	// layer into gopacket.SerializableLayer.
+	if dhcpLayer == nil || dhcpLayer.LayerType() != layers.LayerTypeDHCPv4 {
+		return nil, errInvalidPktDHCP
+	}
+
+	udpLayer := &layers.UDP{
+		SrcPort: dhcpv4.ServerPort,
+		DstPort: dhcpv4.ClientPort,
+	}
+	ipv4Layer := &layers.IPv4{
+		Version:  uint8(layers.IPProtocolIPv4),
+		Flags:    layers.IPv4DontFragment,
+		TTL:      ipv4DefaultTTL,
+		Protocol: layers.IPProtocolUDP,
+		SrcIP:    c.srcIP,
+		DstIP:    peer.yiaddr,
+	}
+
+	// Ignore the error since it's only returned for invalid network layer's
+	// type.
+	_ = udpLayer.SetNetworkLayerForChecksum(ipv4Layer)
+	ethLayer := &layers.Ethernet{
+		SrcMAC:       c.srcMAC,
+		DstMAC:       peer.HardwareAddr,
+		EthernetType: layers.EthernetTypeIPv4,
+	}
+
+	buf := gopacket.NewSerializeBuffer()
+	err = gopacket.SerializeLayers(buf, gopacket.SerializeOptions{
+		FixLengths:       true,
+		ComputeChecksums: true,
+	}, ethLayer, ipv4Layer, udpLayer, dhcpLayer.(gopacket.SerializableLayer))
+	if err != nil {
+		return nil, fmt.Errorf("serializing layers: %w", err)
+	}
+
+	return buf.Bytes(), nil
+}
--- a/internal/dhcpd/conn_unix_test.go
+++ b/internal/dhcpd/conn_unix_test.go
@ -0,0 +1,114 @@
+//go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package dhcpd
+
+import (
+	"net"
+	"strings"
+	"testing"
+
+	"github.com/google/gopacket"
+	"github.com/google/gopacket/layers"
+	"github.com/insomniacslk/dhcp/dhcpv4"
+	"github.com/mdlayher/raw"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestDHCPConn_WriteTo_common(t *testing.T) {
+	respData := (&dhcpv4.DHCPv4{}).ToBytes()
+	udpAddr := &net.UDPAddr{
+		IP:   net.IP{1, 2, 3, 4},
+		Port: dhcpv4.ClientPort,
+	}
+
+	t.Run("unicast_ip", func(t *testing.T) {
+		writeTo := func(_ []byte, addr net.Addr) (_ int, _ error) {
+			assert.Equal(t, udpAddr, addr)
+
+			return 0, nil
+		}
+
+		conn := &dhcpConn{udpConn: &fakePacketConn{writeTo: writeTo}}
+
+		_, err := conn.WriteTo(respData, udpAddr)
+		assert.NoError(t, err)
+	})
+
+	t.Run("unexpected_addr_type", func(t *testing.T) {
+		type unexpectedAddrType struct {
+			net.Addr
+		}
+
+		conn := &dhcpConn{}
+		n, err := conn.WriteTo(nil, &unexpectedAddrType{})
+		require.Error(t, err)
+
+		assert.True(t, strings.Contains(err.Error(), "peer is of unexpected type"))
+		assert.Zero(t, n)
+	})
+}
+
+func TestBuildEtherPkt(t *testing.T) {
+	conn := &dhcpConn{
+		srcMAC: net.HardwareAddr{1, 2, 3, 4, 5, 6},
+		srcIP:  net.IP{1, 2, 3, 4},
+	}
+	peer := &dhcpUnicastAddr{
+		Addr:   raw.Addr{HardwareAddr: net.HardwareAddr{6, 5, 4, 3, 2, 1}},
+		yiaddr: net.IP{4, 3, 2, 1},
+	}
+	payload := (&dhcpv4.DHCPv4{}).ToBytes()
+
+	t.Run("success", func(t *testing.T) {
+		pkt, err := conn.buildEtherPkt(payload, peer)
+		require.NoError(t, err)
+
+		assert.NotEmpty(t, pkt)
+
+		actualPkt := gopacket.NewPacket(pkt, layers.LayerTypeEthernet, gopacket.DecodeOptions{
+			NoCopy: true,
+		})
+		require.NotNil(t, actualPkt)
+
+		wantTypes := []gopacket.LayerType{
+			layers.LayerTypeEthernet,
+			layers.LayerTypeIPv4,
+			layers.LayerTypeUDP,
+			layers.LayerTypeDHCPv4,
+		}
+		actualLayers := actualPkt.Layers()
+		require.Len(t, actualLayers, len(wantTypes))
+
+		for i, wantType := range wantTypes {
+			layer := actualLayers[i]
+			require.NotNil(t, layer)
+
+			assert.Equal(t, wantType, layer.LayerType())
+		}
+	})
+
+	t.Run("non-serializable", func(t *testing.T) {
+		// Create an invalid DHCP packet.
+		invalidPayload := []byte{1, 2, 3, 4}
+		pkt, err := conn.buildEtherPkt(invalidPayload, nil)
+		require.Error(t, err)
+
+		assert.ErrorIs(t, err, errInvalidPktDHCP)
+		assert.Empty(t, pkt)
+	})
+
+	t.Run("serializing_error", func(t *testing.T) {
+		// Create a peer with invalid MAC.
+		badPeer := &dhcpUnicastAddr{
+			Addr:   raw.Addr{HardwareAddr: net.HardwareAddr{5, 4, 3, 2, 1}},
+			yiaddr: net.IP{4, 3, 2, 1},
+		}
+
+		pkt, err := conn.buildEtherPkt(payload, badPeer)
+		require.Error(t, err)
+
+		assert.Empty(t, pkt)
+	})
+}
--- a/internal/dhcpd/dhcpd_test.go
+++ b/internal/dhcpd/dhcpd_test.go
@ -139,7 +139,7 @@ func TestNormalizeLeases(t *testing.T) {
 }

 // cloneUDPAddr returns a deep copy of a.
-func cloneUDPAddr(a *net.UDPAddr) (copy *net.UDPAddr) {
+func cloneUDPAddr(a *net.UDPAddr) (clone *net.UDPAddr) {
 	return &net.UDPAddr{
 		IP:   netutil.CloneIP(a.IP),
 		Port: a.Port,
--- a/internal/dhcpd/options_unix.go
+++ b/internal/dhcpd/options_unix.go
@ -157,8 +157,8 @@ func prepareOptions(conf V4ServerConf) (opts dhcpv4.Options) {
 		dhcpv4.OptionPerformRouterDiscovery.Code(): []byte{1},
 		// The all-routers address is preferred wherever possible, see
 		// https://datatracker.ietf.org/doc/html/rfc1256#section-5.1.
-		dhcpv4.OptionRouterSolicitationAddress.Code(): net.IPv4allrouter.To4(),
-		dhcpv4.OptionBroadcastAddress.Code():          net.IPv4bcast.To4(),
+		dhcpv4.OptionRouterSolicitationAddress.Code(): netutil.IPv4allrouter(),
+		dhcpv4.OptionBroadcastAddress.Code():          netutil.IPv4bcast(),

 		// Link-Layer Per Interface

--- a/internal/dhcpd/v4.go
+++ b/internal/dhcpd/v4.go
@ -19,6 +19,7 @@ import (
 	"github.com/go-ping/ping"
 	"github.com/insomniacslk/dhcp/dhcpv4"
 	"github.com/insomniacslk/dhcp/dhcpv4/server4"
+	"github.com/mdlayher/raw"
 )

 // v4Server is a DHCPv4 server.
@ -955,43 +956,44 @@ func (s *v4Server) packetHandler(conn net.PacketConn, peer net.Addr, req *dhcpv4
 //
 // See https://datatracker.ietf.org/doc/html/rfc2131#section-4.1.
 func (s *v4Server) send(peer net.Addr, conn net.PacketConn, req, resp *dhcpv4.DHCPv4) {
-	var unicast bool
-	if giaddr := req.GatewayIPAddr; giaddr != nil && !giaddr.IsUnspecified() {
+	switch giaddr, ciaddr, mtype := req.GatewayIPAddr, req.ClientIPAddr, resp.MessageType(); {
+	case giaddr != nil && !giaddr.IsUnspecified():
 		// Send any return messages to the server port on the BOOTP
 		// relay agent whose address appears in giaddr.
 		peer = &net.UDPAddr{
 			IP:   giaddr,
 			Port: dhcpv4.ServerPort,
 		}
-		unicast = true
-	} else if mtype := resp.MessageType(); mtype == dhcpv4.MessageTypeNak {
+		if mtype == dhcpv4.MessageTypeNak {
+			// Set the broadcast bit in the DHCPNAK, so that the
+			// relay agent broadcasted it to the client, because the
+			// client may not have a correct network address or
+			// subnet mask, and the client may not be answering ARP
+			// requests.
+			resp.SetBroadcast()
+		}
+	case mtype == dhcpv4.MessageTypeNak:
 		// Broadcast any DHCPNAK messages to 0xffffffff.
-	} else if ciaddr := req.ClientIPAddr; ciaddr != nil && !ciaddr.IsUnspecified() {
+	case ciaddr != nil && !ciaddr.IsUnspecified():
 		// Unicast DHCPOFFER and DHCPACK messages to the address in
 		// ciaddr.
 		peer = &net.UDPAddr{
 			IP:   ciaddr,
 			Port: dhcpv4.ClientPort,
 		}
-		unicast = true
+	case !req.IsBroadcast() && req.ClientHWAddr != nil:
+		// Unicast DHCPOFFER and DHCPACK messages to the client's
+		// hardware address and yiaddr.
+		peer = &dhcpUnicastAddr{
+			Addr:   raw.Addr{HardwareAddr: req.ClientHWAddr},
+			yiaddr: resp.YourIPAddr,
 		}
-
-	// TODO(e.burkov):  Unicast the message to the actual link-layer address
-	// of the client if broadcast bit is not set.  Perhaps, use custom
-	// connection when creating the server.
-	//
-	// See https://github.com/AdguardTeam/AdGuardHome/issues/3443.
-
-	if !unicast {
-		s.broadcast(peer, conn, resp)
-
-		return
+	default:
+		// Go on since peer is already set to broadcast.
 	}

 	log.Debug("dhcpv4: sending to %s: %s", peer, resp.Summary())
-
-	_, err := conn.WriteTo(resp.ToBytes(), peer)
-	if err != nil {
+	if _, err := conn.WriteTo(resp.ToBytes(), peer); err != nil {
 		log.Error("dhcpv4: conn.Write to %s failed: %s", peer, err)
 	}
 }
@ -1029,11 +1031,18 @@ func (s *v4Server) Start() (err error) {

 	s.conf.dnsIPAddrs = dnsIPAddrs

-	laddr := &net.UDPAddr{
-		IP:   net.IP{0, 0, 0, 0},
-		Port: dhcpv4.ServerPort,
+	var c net.PacketConn
+	if c, err = s.newDHCPConn(iface); err != nil {
+		return err
 	}
-	s.srv, err = server4.NewServer(iface.Name, laddr, s.packetHandler, server4.WithDebugLogger())
+
+	s.srv, err = server4.NewServer(
+		iface.Name,
+		nil,
+		s.packetHandler,
+		server4.WithConn(c),
+		server4.WithDebugLogger(),
+	)
 	if err != nil {
 		return err
 	}
--- a/internal/dhcpd/v4_test.go
+++ b/internal/dhcpd/v4_test.go
@ -4,12 +4,11 @@
 package dhcpd

 import (
-	"bytes"
 	"net"
 	"testing"

-	"github.com/AdguardTeam/golibs/netutil"
 	"github.com/insomniacslk/dhcp/dhcpv4"
+	"github.com/mdlayher/raw"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@ -390,67 +389,107 @@ func (fc *fakePacketConn) WriteTo(p []byte, addr net.Addr) (n int, err error) {
 	return fc.writeTo(p, addr)
 }

-func TestV4Server_Send_unicast(t *testing.T) {
-	b := &bytes.Buffer{}
-	var peer *net.UDPAddr
-
-	conn := &fakePacketConn{
-		writeTo: func(p []byte, addr net.Addr) (n int, err error) {
-			udpPeer, ok := addr.(*net.UDPAddr)
-			require.True(t, ok)
-
-			peer = cloneUDPAddr(udpPeer)
-
-			n, err = b.Write(p)
-			require.NoError(t, err)
-
-			return n, nil
-		},
-	}
-
-	defaultPeer := &net.UDPAddr{
-		IP: net.IP{1, 2, 3, 4},
-		// Use neither client nor server port.
-		Port: 1234,
-	}
-	defaultResp := &dhcpv4.DHCPv4{
-		OpCode: dhcpv4.OpcodeBootReply,
-	}
+func TestV4Server_Send(t *testing.T) {
 	s := &v4Server{}

+	var (
+		defaultIP = net.IP{99, 99, 99, 99}
+		knownIP   = net.IP{4, 2, 4, 2}
+		knownMAC  = net.HardwareAddr{6, 5, 4, 3, 2, 1}
+	)
+
+	defaultPeer := &net.UDPAddr{
+		IP: defaultIP,
+		// Use neither client nor server port to check it actually
+		// changed.
+		Port: dhcpv4.ClientPort + dhcpv4.ServerPort,
+	}
+	defaultResp := &dhcpv4.DHCPv4{}
+
 	testCases := []struct {
-		name     string
+		want net.Addr
 		req  *dhcpv4.DHCPv4
-		wantPeer net.Addr
+		resp *dhcpv4.DHCPv4
+		name string
 	}{{
-		name: "relay_agent",
-		req: &dhcpv4.DHCPv4{
-			GatewayIPAddr: defaultPeer.IP,
-		},
-		wantPeer: &net.UDPAddr{
-			IP:   defaultPeer.IP,
+		name: "giaddr",
+		req:  &dhcpv4.DHCPv4{GatewayIPAddr: knownIP},
+		resp: defaultResp,
+		want: &net.UDPAddr{
+			IP:   knownIP,
 			Port: dhcpv4.ServerPort,
 		},
 	}, {
-		name: "known_client",
-		req: &dhcpv4.DHCPv4{
-			GatewayIPAddr: netutil.IPv4Zero(),
-			ClientIPAddr:  net.IP{2, 3, 4, 5},
+		name: "nak",
+		req:  &dhcpv4.DHCPv4{},
+		resp: &dhcpv4.DHCPv4{
+			Options: dhcpv4.OptionsFromList(
+				dhcpv4.OptMessageType(dhcpv4.MessageTypeNak),
+			),
 		},
-		wantPeer: &net.UDPAddr{
-			IP:   net.IP{2, 3, 4, 5},
+		want: defaultPeer,
+	}, {
+		name: "ciaddr",
+		req:  &dhcpv4.DHCPv4{ClientIPAddr: knownIP},
+		resp: &dhcpv4.DHCPv4{},
+		want: &net.UDPAddr{
+			IP:   knownIP,
 			Port: dhcpv4.ClientPort,
 		},
+	}, {
+		name: "chaddr",
+		req:  &dhcpv4.DHCPv4{ClientHWAddr: knownMAC},
+		resp: &dhcpv4.DHCPv4{YourIPAddr: knownIP},
+		want: &dhcpUnicastAddr{
+			Addr:   raw.Addr{HardwareAddr: knownMAC},
+			yiaddr: knownIP,
+		},
+	}, {
+		name: "who_are_you",
+		req:  &dhcpv4.DHCPv4{},
+		resp: &dhcpv4.DHCPv4{},
+		want: defaultPeer,
 	}}

 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			s.send(defaultPeer, conn, tc.req, defaultResp)
-			assert.EqualValues(t, defaultResp.ToBytes(), b.Bytes())
-			assert.Equal(t, tc.wantPeer, peer)
-		})
+			conn := &fakePacketConn{
+				writeTo: func(_ []byte, addr net.Addr) (_ int, _ error) {
+					assert.Equal(t, tc.want, addr)

-		b.Reset()
-		peer = nil
+					return 0, nil
+				},
 			}
+
+			s.send(cloneUDPAddr(defaultPeer), conn, tc.req, tc.resp)
+		})
+	}
+
+	t.Run("giaddr_nak", func(t *testing.T) {
+		req := &dhcpv4.DHCPv4{
+			GatewayIPAddr: knownIP,
+		}
+		// Ensure the request is for unicast.
+		req.SetUnicast()
+		resp := &dhcpv4.DHCPv4{
+			Options: dhcpv4.OptionsFromList(
+				dhcpv4.OptMessageType(dhcpv4.MessageTypeNak),
+			),
+		}
+		want := &net.UDPAddr{
+			IP:   req.GatewayIPAddr,
+			Port: dhcpv4.ServerPort,
+		}
+
+		conn := &fakePacketConn{
+			writeTo: func(_ []byte, addr net.Addr) (n int, err error) {
+				assert.Equal(t, want, addr)
+
+				return 0, nil
+			},
+		}
+
+		s.send(cloneUDPAddr(defaultPeer), conn, req, resp)
+		assert.True(t, resp.IsBroadcast())
+	})
 }